The present application claims priority under 35 U.S.C. xc2xa7119 to Japanese Patent Application No. 2001-080944, filed on Mar. 21, 2001, the entire contents thereof are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a GPS receiving apparatus. More particularly, the present invention relates to a GPS receiving apparatus capable of improving measurement accuracy by selectively using only some of the GPS signals transmitted by a number of satellites.
2. Description of the Prior Art
As a navigation system employed in a moving body such as a car and a vessel, in Japanese Patent Laid-open No. Hei 4-324384, there has been disclosed an apparatus for determining the position of the moving body on the basis of GPS (Global Positioning System) signals, with each signal being an electric received by the apparatus from an artificial satellite. The GPS receiving apparatus for determining a position receives GPS signals from three or more artificial satellites. It then computes distance data and altitude data from navigation messages transmitted by the artificial satellites, in order to find the position of the moving body.
As a GPS antenna, a planar antenna is adopted. The planar antenna is known as the so-called patch antenna having a disc-like shape with a diameter in the range 20 to 60 mm. In such a planar antenna, a number of antenna elements can be integrated so that its directivity in the vertical direction can be increased substantially. In addition, the planar antenna has a diversity configuration to provide a good reception condition.
When an electric wave transmitted by a satellite having a small angle of elevation such that the position of the satellite close to the horizon, the navigation system utilizing the GPS is adversely affected by a multipath, an ionized-layer delay, or the like. When this occurs, it is generally known that the precision of the system deteriorates. On the other hand, the planar antenna described above has a high directivity in the vertical direction, and a low sensitivity in the horizontal direction. Thus, a navigation system using such a planar antenna is only minimally affected by the multipath, the ionized-layer delay, or the like.
Given the above characteristics of a planar antenna, problems may occur if a planar antenna is employed in the GPS system of a moving body which changes its posture considerably during a movement. Despite the strong directivity of the planar antenna, when the moving body is put into an inclined posture, the electric wave output of a satellite having a small angle of elevation, unavoidably causes the precision of the GPS system to deteriorate. Examples of such a moving body where this problem occurs are a motor bicycle and a buggy car.
Similarly, if a non-directional antenna is employed, other problems occur. If a non-directional antenna is used to detect an electric wave in the vertical direction, and the moving body is in an inclined posture, such an antenna would receive not only an electric wave in the vertical direction, but also an electric wave in the horizontal direction.
Thus, with either of these conventional devices, the technological problems of deteriorating precision remain.
It is thus an object of the present invention to address the problems described above, by providing a GPS receiving apparatus capable of selectively acquiring only a GPS signal included in an electric wave transmitted by a satellite with a large angle of elevation.
In order to achieve the object described above, the present invention provides a GPS receiving apparatus for receiving GPS signals transmitted by a plurality of satellites. The GPS receiving apparatus includes an antenna for receiving the electric waves from the satellites; an elevation-angle-computing unit for finding an elevation angle of each satellite on the basis of the GPS signal received from the satellite; and a comparison unit for comparing the elevation angle of each of the satellite with a predetermined reference elevation angle. Only GPS signals transmitted by those satellites having an elevation angle greater than the predetermined reference elevation angle are selectively used.
In accordance with the characteristics described above, only GPS signals transmitted by those satellites having an elevation angle greater than the predetermined reference elevation angle are selectively used. Thus, a position can be computed with a high degree of precision without being adversely affected by a multipath, an ionized-layer delay, or the like.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.